Multiplexer with level shift capabilities

ABSTRACT

An integrated circuit having system logic with programmable elements, decoding logic coupled to the programmable elements for addressing the programmable elements and a plurality of input/output buffer circuits for passing signals between the system logic and the exterior of the integrated circuit through input/output terminals is disclosed. Each input/output buffer circuit comprises an output driver stage having an output terminal connected to an input/output terminal; and a plurality of cells, each cell having a multiplexer, a flip-flop connected to an output terminal of the first multiplexer for storing a signal from the first multiplexer, a latch connected to an output terminal of the flip-flop for storing a signal from the flip-flop, and a second multiplexer connected to an output terminal of the latch. The cells connected to each other and cells of other input/output buffer circuits from an output terminal of the flip-flop of one cell to a first input terminal of a first multiplexer of another cell for serial scanning of signals through the cells to test the system logic. Control lines are connected to the output terminals of the latch of the cells and to the decoding logic coupled to the programmable elements so that the programmable elements can be addressed for programming by serially scanning control signals through the cells.

This is a Division of Application Ser. No. 08/078,692 filed Jun. 17,1993, which is a Division of application Ser. No. 07/718,677 filed Jun.21, 1991 , which issued as U.S. Pat. No. 5,221,865.

BACKGROUND OF THE INVENTION

The present invention is related to input/output buffer circuits inintegrated circuits and, more specifically, to buffer circuits whichhave various optionally programmable operating characteristics and whichhave certain test capabilities.

In an integrated circuit, input output buffer circuits lie electricallybetween the rest of the integrated circuit and the external environment,i.e., the system in which the integrated circuit is placed. As the nameimplies, these circuits "buffer" or condition signals from the externalenvironment to the integrated circuit and signals from the integratedcircuit to the external environment. In nearly all integrated circuitsthese buffer circuits are designed with a particular externalenvironment in mind. Thus a redesign is required if the integratedcircuit is to be relocated in a different system having differentrequirements. It is highly desirable that input/output buffer circuitsbe easily adaptable to different external environments.

Furthermore, a recent requirement upon input/output buffer circuits istestability. As integrated circuits and the systems in which they arelocated have become more complex and densely packed, the testing of theintegrated circuits have become more complex and difficult. One proposedsolution is the IEEE standard 1149.1 for boundary scan testing in whichthe input/output buffer circuits form a serial scan chain over whichtest data can be passed. Test data can be scanned into the integratedcircuit over the serial chain between the buffer circuits, processed bythe integrated circuit and then scanned out over the chain. The changesto the processed test data yield the desired test information about theintegrated circuit.

However, the IEEE standard 1149.1 requires circuits in the input/outputbuffer to perform the scanning function which are additional to thecircuits for input and output functions. This added "overhead" to aninput/output buffer circuit occupies valuable space on the semiconductorsubstrate on which the integrated circuit is formed and typically slowsthe operating speed of the buffer circuit.

It is thus desirable that an input/output buffer circuit have circuitswhich can perform the input/output and the test functions in aconsolidated fashion to reduce the space occupied by the buffer circuitand which can function at high speeds so as to avoid any denigration ofperformance.

The present invention is able to achieve all these goals and more.

SUMMARY OF THE INVENTION

The present invention provides for an integrated circuit having systemlogic with programmable elements, decoding logic coupled to theprogrammable elements for addressing the programmable elements and aplurality of input/output buffer circuits for passing signals betweenthe system logic and the exterior of the integrated circuit throughinput/output terminals. Each input/output buffer circuit comprises anoutput driver stage having an output terminal connected to aninput/output terminal; and a plurality of cells, each cell having afirst multiplexer, a flip-flop connected to an output terminal of thefirst multiplexer for storing a signal from the first multiplexer, alatch connected to an output terminal of the flip-flop for storing asignal from the flip-flop, and a second multiplexer connected to anoutput terminal of the latch. The cells connected to each other andcells of other input/output buffer circuits from an output terminal ofthe flip-flop of one cell to a first input terminal of a firstmultiplexer of another cell for serial scanning of signals through thecells to test the system logic. Control lines are connected to theoutput terminals of the latch of the cells and to the decoding logiccoupled to the programmable elements so that the programmable elementscan be addressed for programming by serially scanning control signalsthrough the cells.

Each input/output buffer circuit has a programming unit which isconnected to the control lines from the latches of the cells. Throughthese control lines the programming unit can also be programmed to setthe drive characteristics of the output driver stage.

The input/output buffer circuit has also many other useful features,including special test pads which are connected to the bonding padswhich handle the serial scan test signals and power. Located in thecorner(s) of the integrated circuit die, the test pads can be probed fortesting of the integrated circuit while the die is still part of a waferwithout touching the bonding pads.

Other useful features include special circuits which can act asmultiplexers and level-shifters without any significant loss in speed.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed understanding of the present invention may be attained by aperusal of the following Description of the Specific Embodiments withreference to the drawings below.

FIG. 1 is block diagram of an input/output buffer circuit according tothe present invention.

FIG. 2 is a circuit schematic diagram of the output stage of FIG. 1.

FIG. 3 is a circuit schematic diagram of a programmable multiplexer ofFIG. 1.

FIG. 4 is a schematic diagram of a Schmitt trigger circuit which may beused in the multiplexer in FIG. 3.

FIG. 5 is a circuit schematic diagram of another level-shiftingmultiplexer of FIG. 1.

FIG. 6 is a circuit schematic diagram of the programming unit of FIG. 1.

FIG. 7A illustrates how the input/output buffer circuit of the presentinvention may be connected to other such circuits in an integratedcircuit; FIG. 7B schematically illustrates a cross-bar switch of FIG.7A.

FIG. 8 illustrates an alternative way of connecting the input/outputbuffer circuits of the present invention in an integrated circuit.

FIG. 9 illustrates a test pad arrangement according to the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is an general block circuit diagram of the an input/output buffercircuit according to the present invention. It should be noted thatalthough the buffer circuit is described in the context of fieldprogrammable gate array (FPGA) integrated circuits, one skilled in thefield of integrated circuit design would readily appreciate theapplicability of the present invention to other types of integratedcircuits.

As explained in greater detail below, the input/output buffer circuitcan act as an output buffer to transmit data signals from the rest(interior) of the integrated circuit to the outside environment. Theinput/output buffer circuit can act as an input buffer to transmit datasignals from the outside environment to the interior, or system logic,of the integrated circuit. As the data signals move through the buffercircuit, the buffer circuit also shifts the voltage levels of thesignals for operation in the interior of the integrated circuit and foroperation in the outside environment.

The input/output buffer circuit can also operate under the IEEE standard1149.1. In this mode of operation the buffer circuit, which is connectedto other input/output buffer circuits, can transmit signals between thebuffer circuits in a serial chain. In a sense, signals in this mode oftransport travel perpendicularly to the direction of data signals to andfrom the interior of the integrated circuit. These serial chain signalsmay be test data signals under the IEEE test standard, or may be signalsuseful in programming the integrated circuit, including the input/outputbuffer circuit itself.

Besides signals from the scanning operation along the chain ofinput/output buffer circuits, the buffer circuit can acquire signalsfrom locations in the system logic of the integrated circuit other thanwhere the test data signals are acquired, from the external environment,and from the input/output buffer circuit itself. Such flexibilitypermits the input/output buffer circuit to perform a host of functions,as explained below.

The buffer circuit of FIG. 1 is generally organized with a programmingunit 50 and a decoder 51 to program the unit 50, an output driver stage10, and three cells, which are combinations of a multiplexer, aflip-flop, a latch and a second multiplexer.

The programming unit 50 contains antifuses which are programmed to setvarious control lines emanating from the unit 50. The programming unit50 can be used to set the operational characteristics of the outputsignals from the input/output buffer circuit and to handle differentoperational characteristics of the input signals to the input/outputbuffer circuit. For example, once programmed, the unit 50 sends controlsignals to the output stage 10 to select a particular drive current andslew rate during an output operation.

A control signal on the line 70 through a NAND gate 57 sends a signal onthe line 60 to the output stage 10 so that the output drive current isset at a particular value, 4 mA, in this case. A control signal on theline 71 passes through a NOR gate 55 to generate a control signal on theline 61 so that the output stage 10 generates a higher output current, 8mA. An output current of 12 mA is also possible with a combination ofcontrol signals. The programming unit 50 also controls the slew rate ofthe output driver stage 10 by a control line 76.

Other control lines from the programming unit 50 include a control line75 connected to a control line 64, a level shifter 58, a multiplexer 23and a line 24. When set, the control line 75 disables the output driverstage 10, i.e, the stage 10 is placed in a state of high impedance. Acontrol line 69, which is connected to a data out line 35 (and line 34)can pull the line 35 high or low depending upon how the programming unit50 is set. Thus, together with the operation of the control line 75,which can be set to carry clock signals, as explained below, the outputdriver stage 10 can be set in an "open source" or "open drain" mode,i.e., either logic high/high impedance or logic low/high impedance.

Control lines 73 and 74 are connected to input terminals of multiplexers30 and 40 respectively. These control lines 73 and 74 provide access tothe programmable bits in the unit 50. As explained in greater detailbelow, these programmable bits may used by the manufacturer or user ofthe integrated circuit to store information.

Control lines 77, 78 and 79 carry signals from the programming unit 50to control the mode of operation of a multiplexer 43, as explainedbelow.

In its output operation, the output driver stage 10 transmits signals onthe line 34 from a multiplexer 33 to an input/output terminal 11.Typically the terminal 11 is an input/output bonding pad of theintegrated circuit. Depending upon the selection of the multiplexer 33,the signal on the line 34 could be data signal on a data out line 35through a latch 54 or a signal from a latch 32. As explained above, theoutput driver stage 10 is programmable so that different currents andslew rates are available through the control lines 60, 61 and 76 todrive the terminal 11 for output operation or to place the terminal 11in a high impedance state through the line 24.

The first cell, or multiplexer-flip-flop-latch-multiplexer combination,has a 4-to-1 multiplexer 20 which has its output terminal connected toan input terminal of a flip-flop 21. The output terminal of theflip-flop 21 is connected to an input terminal of a latch 22 which, inturn, has its output terminal connected to an input terminal of the2-to-1 multiplexer 23. The output terminal of the multiplexer 23 isconnected by the line 24 to an input terminal of the output stage 10.

The four input terminals of the multiplexer 20 are connected to anoutput terminal of the level shifter 58, a logic "1" state generator,here the voltage supply at V_(cc), a line 28 which is connected to anode in the interior of the integrated circuit and a line 26 connectedto an output terminal of a flip-flop 31 belonging to the secondmultiplexer-flip-flop-latch-multiplexer combination described below. Themultiplexer 20 is also connected to control lines 80A and 80B (two ofthree Select Data Register control lines), which determine which of thefour input terminals is selected.

The flip-flop 21 is connected to two clock lines 81A and 81B (Clock Datacontrol lines), which normally run in complementary fashion. Asdescribed later, the clock lines 81A and 81B can also operate in othermodes including a pass-through mode by which signals pass directly fromthe input terminals to the output terminals of the flip-flops 21.Besides the input terminal of the latch 22, the output terminal of theflip-flop 21 is connected to an input terminal of a 2-to-1 multiplexer59. The second input terminal of the multiplexer 59 is connected to aline 27 which is connected to the output terminal of the latch 22 and toa node in the interior of the integrated circuit. The latch 22 is alsoconnected a control line 82 (Update Register control line).

Besides the output terminal of the latch 22, the 2-to-1 multiplexer 23of the first combination is connected the output terminal of the levelshifter 58 and a control line 83 (Output Mode control line) over whichselection signals are transmitted.

Similarly, the second cell has a 4-to-1 multiplexer 30 which has itsoutput terminal connected to an input terminal of the flip-flop 31. Theoutput terminal of the flip-flop 31 is connected to an input terminal ofa latch 32 which, in turn, has its output terminal connected to an inputterminal of a 2-to-1 multiplexer 33. The output terminal of themultiplexer 33 is connected by the line 34 to an input terminal of theoutput stage 10.

The four input terminals of the multiplexer 30 are connected to anoutput terminal of the latch 54, the control line 73, a line 38connected to a node in the interior of the integrated circuit, and aline 36 connected to the output terminal of a flip-flop 41 belonging tothe third combination. Like the multiplexer 20, the multiplexer 30 isconnected to the control lines 80A and 80B.

The flip-flop 31 is connected to the control lines 81A and 81B like theflip-flop 21 of the first combination. Similarly, the latch 32 isconnected to the control line 82 like the latch 22 and the 2-to-1multiplexer 33, which has another input terminal connected to the outputterminal of the latch 54, is connected to the control line 83 like themultiplexer 23.

The third cell is somewhat different from the other two cells. Like theother multiplexers of the first two cells, a 4-to-1 multiplexer 40 ofthe third cell has its output terminal connected to a flip-flop 41 whichhas its output terminal connected to an input terminal of a latch 42.The output of the latch 42 is connected to an input terminal of aprogrammable 2-to-1 multiplexer 43.

The four input terminals of the multiplexer 40 are connected to a datainput line 45 leading to the system logic of the integrated circuit, thecontrol line 74 from the programming unit 50, a line 48 from a node inthe interior of the integrated circuit, and a line 46. As in the case ofthe multiplexers 20 and 30, the multiplexer 40 is connected to thecontrol lines 80A and 80B for selection of input terminals. Similarlythe flip-flop 41 is connected to the control lines 81A and 81B, and thelatch 42 is connected to the control line 82.

The multiplexer 43 differs from the multiplexers 23 and 33. The outputterminal of the multiplexer 43 is connected, not to the output driverstage 10, but rather to the interior of the integrated circuit by theline 45. One input terminal of the multiplexer 43 is connected to theoutput terminal of the latch 42, but the second input terminal isconnected to the input/output terminal 11 by a line 44. The multiplexer43 is connected to a control line 84 (Input Mode control line) carriessignals to put multiplexer 43 into a high impedance condition. Themultiplexer 43 is also connected to the control lines 62, 63 and 77.Signals on the control line 62 determine whether the input terminalconnected to the terminal 11 or the input terminal connected to thelatch 42 is selected. If the terminal 11 is selected, the controlsignals on the line 63 determine whether the multiplexer 43 operates atTTL (Transistor-Transistor Logic) levels or at CMOS (ComplementaryMetal-Oxide-Semiconductor) transistor levels. Signals on the controlline 77 determine whether the terminal 11 has a pullup or not.

The control lines 80 (80A-80C), 81 (81A-81B), 82, 83 and 84 carrysignals from control logic in the integrated circuit. Such logic may bedesigned by a person skilled in integrated circuit design to operatewith the present invention as described above and according to therequirements of the particular integrated circuit. However, the presentinvention is also being described in the context of the IEEE 1149.1standard. Thus the control logic is constrained by the specifications ofthe standard.

Thus on an integrated circuit using the input/output buffer circuit ofthe present invention, the buffer circuit is typically replicated foreach input/output terminal. The input buffer circuits are connected incommon to the previously described control lines 80-85. Furthermore, theline 46, which is connected to an input terminal of the multiplexer 40,of one input/output buffer circuit is connected to the correspondingline 16 of a neighboring input/output buffer circuit. Likewise, the line16 of the first input/output buffer circuit is connected to thecorresponding line 46 of a second neighboring input/output buffercircuit.

During operation, signals are transferred through the input/outputbuffer circuit of FIG. 1 in many ways. For incoming data signals, thedata signal from the terminal 11 travels through the line 44 to themultiplexer 43. With an input mode selected by a control signal on theline 84, the data signal passes through the multiplexer 43 to the line45 and on the rest of the integrated circuit. Output data signals travelfrom the interior of the integrated circuit on the line 35 to the latch54, which is clocked by signals on a line 39. The output data signalsfrom the latch 54 are passed to the multiplexer 33, which by an outputmode selection signal on the control line 83 sends the output datasignals to the output driver stage 10. It should be noted that the latch54 can be set to hold or pass the data signals by a signal on thecontrol line 72 from the programming unit 50.

In the particular embodiment of the described input/output buffercircuit, the interior, or system logic, of the integrated circuitoperates at reduced CMOS logic levels, i.e., between 0 to +3.5 volts.The power supply is accordingly reduced. For communicating to theexterior world, the interior signals are level-shifted from reduced CMOSto full CMOS logic levels, i.e., 0 to ±5 volts. Hence as shown in FIG.1, the level shifter 58 is placed between the control line 64 and themultiplexer 23. NAND gate 57, NOr gate 55, latch 54 and NAND gates 52and 53 also perform level-shifting functions. As described blow, signalsalso move through the input/output buffer circuit for serial scantesting. These signals are at full CMOS logic levels as specified in theIEEE 1149.1 standard.

Signals can also move through the input/output buffer circuit in theIEEE standard 1149.1 test mode. In this mode, test data signals travelto the line 46 (TDI in the terminology of the standard) from thecorresponding line 16 (TDO in the terminology of the standard) of aneighboring input/output buffer circuit. The test data can be seriallyscanned through each input/output buffer circuit by the movement of datathrough the multiplexer 40, the flip-flop 41, and the line 36 to themultiplexer 30. Then, the data continues through the flip-flop 31 andline 26 to the multiplexer 20. From the multiplexer 20 the data travelsthrough the flip-flop 21 and the multiplexer to the next input/outputbuffer circuit on the line 16.

For a test of the integrated circuit the test data are scanned inserially as described, sent into the system logic of the integratedcircuit on the line 45 of each input/output buffer circuit, processed bythe integrated circuit, and read out on the line 35 of each input/buffercircuit. The processed test data are then scanned out serially andcompared with expected test data. All of these operations are describedin the specification of the IEEE 1149.1 standard.

With the present invention, signals can be serially scanned through theinput/output buffer circuits for purposes other than test. For example,control signals may be moved along the serial scan path described aboveuntil the each control signal is in the flip-flops 21, 31, and 41 ofeach input/output buffer circuit. Then the latches 22, 32 and 42 areenabled to respectively transmit the signals on the lines 27, 37 and 47to the interior of the integrated circuit, which contains a programmablelogic array. These control signals are received by decoding logic (notshown) which address the programmable elements in the array forprogramming. The programmable logic array and its programming operationcontemplated by this embodiment of the present invention is described inU.S. Ser. No. 07/671,222, entitled "A FIELD PROGRAMMABLE LOGIC ARRAY,"filed Mar. 18, 1991 by Laurence H. Cooke and David Marple, and assignedto the present assignee.

Besides programming the logic array of the interior, control signals inthe latches 22, 32 and 42 are used to program the programming unit 50 inthe input/output buffer circuit itself. Each of the control lines 27, 37and 47 leads not only to the interior of the integrated circuit but alsoto the decoder 51 which addresses the particular programmable elementsin the unit 50. Together with a control line 86, the lines 27, 37, and47 allows the decoder 51 to perform a 4-to-1 selection for programmingthe unit 50. The control line 50 is connected to a decoder for theInstruction Register defined by the IEEE 1149.1 standard. Since theInstruction Register itself is loaded by serial scanning through the TDIterminal, the programming of the unit 50 is defined completely by theserial scanning signals. The self-programming operation is detailed inthe description of the programming unit 50 below.

Of course, one skilled in integrated circuit design would readilyappreciate other applications besides the programming of theprogrammable logic array described above or the self-programming of theinput/output buffer circuit.

It should be noted that signals, data or otherwise, may be received bythe input/output buffer circuit from the terminal 11, through themultiplexer 43 and line 45. From the line 45, the signals are receivedby the multiplexer 40 and stored in the flip-flop 41. From here thesignals may be serially scanned as described for the test data signals.

The input/output buffer circuit may also receive signals in other ways.The lines 28, 38 and 48 are connected to various nodes in the systemlogic of the integrated circuit. By appropriate signals on the controllines 80A and 80B, the logic states at these nodes are read out throughthe multiplexers 20, 30 and 40. These signals may then be scanned outserially as described for the IEEE standard test data or read outsequentially through the flip-flop 31, latch 32, multiplexer 33 andoutput stage 10 of each of the input/output buffer circuits. One skilledin the field of integrated circuit design should readily appreciate thebenefits of such direct access to the system logic of an integratedcircuit.

The multiplexers 30 and 40 also have one of their input terminalsconnected to the programming unit 50 by the lines 73 and 74. Upon theproper signals on the control lines 80, a bit of information stored inthe programming unit 50 is respectively loaded into the flip-flops 31and 41. At the same time a logic "1" from the input terminal of themultiplexer 20 connected to the V_(cc) power supply is loaded into theflip-flop 21. The two storage locations in the programming unit 50 arepart of registers which are defined in the IEEE test specification. Eachinput/output buffer circuit contains a two-bit portion of registersincluding the Identification Code Register, User Code Register, and UserTest Register. Furthermore, the two storage locations may be used forother registers, such as a manufacturing code register.

Thus the input/output buffer circuits are placed around the periphery ofthe die on which the integrated circuit is formed. The circuits areconnected together to form the scanning chain as described previously.As shown in FIG. 7A, a cross-bar switch 90, which is detailed at thelogic level in FIG. 7B, is placed between every 16 input/output buffercircuits, which form the registers 91 with 32 storage locations in the16 programming units 50. The cross-bar switches 90 receive the datasignals on the serial scan chain and, responsive to signals on controlline 93A, 93B, etc., either passes the signals to the next 16input/output buffer circuits in the scan chain or sends the signals tothe next cross-bar switch 90. Together with the operation of thecross-bar switches 90 and responsive to selection signal on the controlline 93, the output multiplexer 92 selects corresponding register datato be read out through the multiplexer output terminal 94.

It should be noted that the IEEE test specification requires that theIdentification Code Register and other registers be 32-bits long.Accordingly the input/output buffer circuit uses only two of the threepossible bits. When reading the two bits from the programming unit 50through the multiplexers 30 and 40, the corresponding input line of thefirst cell multiplexer 20 is set to logic "1". When the 32 bits of datafrom neighboring input/output buffer circuits are serially scanned, thecontrol line 80C operates so that the multiplexer 59 selects inputsignals on the line 26 and avoids the logic "1" held by the flip-flop21. In this manner each input/output buffer circuit operates with theserial data path from the line 46, through the multiplexer 40, throughthe flip-flop 41, out to the line 36, through the multiplexer 30 andflip-flop 31, output to the line 26, through the multiplexer 59 and outto the line 16 to the next buffer circuit.

An alternative way of connecting the input/output buffer circuits for aserial scanning chain around the die periphery is illustrated in FIG. 7.In this connection the (TDO) output terminal of each register 91, i.e.,group of 16 connected input/output buffer circuits, is connected to aninput terminal of a large output multiplexer 95. Responsive to selectionsignals on control lines 96, the multiplexer 95 selects the register 91from which to read out the data signals. Compared to the connectionillustrated in FIG. 6A, the connection of FIG. 8 requires moreconducting lines as one proceeds down the chain toward the outputmultiplexer 95. This is in contrast to the two conducting paths for thedata signals in FIG. 7A.

FIG. 2 is a detailed schematic diagram of the output driver stage 10.The output driver stage 10 is divided up into three parallel-connectedparts, 10A, 10B and 10C. Each part 10A-10C is identical to the other.The large driver transistors are of each part 10A-10C to drive theterminal 11 with 4 mA of drive current. If two parts are enabled, theterminal 11 is driven with 8 mA of drive current. When all three partsare selected, then the terminal 11 is driven with 12 mA of current.

Of course, each part 10A-10C may be designed to drive with differentcurrents. By selecting different combinations of parts, differentamounts of drive currents are possible. The output driver stage 10 mayalso be designed with more (or less) than three parts.

Since the three parts 10A-10C are the same, only part 10A is discussedin detail.

As shown in FIG. 2, the data signal line 34 is connected to the gateelectrodes of a complementary pair of data transistors, PMOS transistor108 and NMOS transistor 109. The source electrode of the PMOS transistor108 is connected to a power supply at V_(cc) and its drain electrode isconnected to the gate electrode of one of the complementary pair ofdriver transistors, PMOS transistor 100. The drain electrode of the NMOStransistor 109 is connected to the gate electrode of the NMOS drivertransistor 101 and its source electrode to a power supply at ground,GND. The drain electrodes of both driver transistors 100 and 101 arecommonly connected to the terminal 11. The source electrode of the PMOSdriver transistor 100 is connected to a second power supply at V_(cc)and the source electrode of the NMOS driver transistor 101 is connectedto a second power supply at ground GND0. While nominally at V_(cc) andground respectively, these second power supplies are electricallyseparated from the V_(cc) and ground power supplies for the reasonsbelow. As shown in FIG. 1, these second power supplies are distributedto each input/output buffer circuit by conducting wires.

The part 10A also has complementary transistors, PMOS transistor 105 andNMOS transistor 107, each of which is respectively connected by theirdrain electrodes to the gate electrodes of the driver transistors 100and 101. The gate electrodes of NMOS transistor 107 (and a PMOStransistor 112) is connected to the output terminal of a NAND gate 113which receives the output enable line 24 and the 4 mA current driveenable line 60 as inputs. The PMOS transistor 105 (and a NMOS transistor106) is connected to the output terminal of the NAND gate 113 through aninverter 111. The NMOS transistor 106 has one source/drain electrodeconnected to the gate electrode of the PMOS driver transistor 100 andthe other source/drain electrode to the gate electrode of the NMOSdriver transistor 101. The PMOS transistor 112 has a drain electrodeconnected to the gate electrode of the driver transistor 101. The sourceelectrode of the transistor 112 is connected to the drain electrodes ofthe two parallel PMOS transistors 110 and 114. Both PMOS transistors 110and 114 have their source electrodes connected to the gate electrode ofthe PMOS driver transistor 100 and drain electrode of the PMOS datatransistor 108. The gate electrode of the PMOS transistor 110 isconnected to the input/output terminal 11, while the gate electrode ofthe PMOS transistor 114 is connected to the slew rate control line 76.

A complementary pair of driver transistor control transistors, PMOStransistor 102 and NMOS transistor 103 are connected, respectively, tothe PMOS driver transistor 100 and NMOS driver transistor 101. The PMOStransistor 102 has its source electrode connected to the voltage supplyat V_(cc) and its drain electrode connected to the gate electrode of thedriver transistor 100. The gate electrode of the transistor 102 isconnected to the source electrode of the driver transistor 100 and tothe power supply at V_(cc0), which is nominally at the same voltage atV_(cc) but electrically uncoupled from the V_(cc) power supply.

Likewise, the NMOS transistor 103 has its source electrode connected tothe power supply at ground (GND) and has its drain electrode connectedto the gate electrode of the NMOS driver transistor 101. The gateelectrode of the transistor 103 is connected to the source electrode ofthe PMOS driver transistor 101 and to the power supply at GND0. ThisGNDO power supply is also nominally at ground but is electricallyuncoupled from the GND power supply.

If either of the output enable line 24 or the 4 mA enable line 60 is alogic "0" the part 10A is in a high impedance state. The PMOS transistor105 and NMOS transistor 107 are turned on, respectively turning off thePMOS driver transistor 100 and the NMOS driver transistor 101. The part10A of the output stage 10 is effectively turned off. If the 8 mAcurrent drive enable line 61 is also disabled, i e , logic "0" then theparts 10B and 10C are also disabled and the output stage 10 is a highimpedance state. The output terminal 11 is electrically uncoupled.

When the output enable line 24 and the 4mA enable line are both logic"1" the part 10A becomes conducting. The PMOS transistor 112 and NMOStransistor 106 are turned on, and the transistors 105 and 107 are turnedoff. The data transistors 108 and 109, in response to the data signal onthe line 34, control the output driver transistors 100 and 101 and thelevel of the output signal at the terminal 11.

It should be noted that the NMOS transistor 106 and PMOS transistors112, 110 and 114 are sized smaller than the data transistors 108 and109. These smaller transistors are thus considerably more resistive whenconducting. The result is that voltage on the gate electrode of the PMOSdriver transistor 100 can rise quickly but fall slowly and the gateelectrode of the NMOS driver transistor 101 can fall quickly but riseslowly. This design allows the very large output driver transistors 100and 101 to turn off quickly and to turn on slowly. This avoids orconsiderably lessens the switching current through the two transistors100 and 101 which results if both transistors are simultaneouslyconducting.

This design technique of using the transistor size differential to allowthe large driver transistors of an output circuit to turn off quicklybut to turn on slowly, also alleviates the problem of "ground bounce."Ground bounce is the term for the problem of transient voltage rises atan integrated circuit output lead at a low voltage state caused by therapid switching of other output leads of the integrated circuit. Throughthe parasitic inductive coupling of the output leads of the integratedcircuit and its package, these transient voltages, which are typicallyworse for switching from high to low voltages, may cause the localground power supply of the integrated circuit to rise. This rise inground raises the voltage levels of those output leads at a low level.If the rise is large enough, the voltage levels may be improperly sensedas being at a high voltage level. Ground bounce can also be caused bythe simultaneous switching of the output driver transistors which pullthe output terminal up and down in voltage.

However, a straightforward transistor size differential technique hassome shortcomings. While the size of the transistors can be designed fora certain set of conditions so that the output driver transistors turnon slowly enough, variations in the conditions, such as temperature,supply voltages, and even processing of the integrated circuit, mayrender the operation of the output driver transistors too slow.

In the present invention the output stage 10 has feedback to optimizethe switching operation of the output driver transistors 100 and 101while avoiding ground bounce. The PMOS transistor 110, which has itsgate electrode connected to the terminal 11, senses the voltage at theterminal. The lower the terminal 11 drops in voltage, the greater thetransistor 110 turns on and the faster the gate electrode of the drivertransistor 101 is pulled up. The PMOS transistor 101 turns on faster.This feedback provides for the terminal 11 to be pulled down faster asthe rate of current increase reduces in magnitude. The output terminal11 is discharged faster after the initial ground bounce spike has passedand results in less output buffer delay.

Besides the transistor size differentials, the output driver stage 10also has another immunization measure against ground bounce. Each unit10A-10C has driver control transistors 102 and 103. The NMOS drivercontrol transistor 103 senses the local ground voltage, GNDO, andprovides feedback which tends to pull down the voltage on the gateelectrode of the NMOS driver transistor 101 to reduce its conductionwhen GNDO rises due to ground bounce. This feedback operation is mostactive under conditions which result from a strong effect of thepulldown action of the transistor 101, i.e., a severe ground bounce. Thetransistor 103 operates as a clamp to shut off the driver transistor 101as to limit the rise in GNDO. The transistor 103 has little effect underconditions where switching is slower and ground bounce is not a problem.This allows the gate electrode of the NMOS driver transistor 101 to risequickly and preserves the speed of the output stage 10.

It should be noted that the transistor 103 can operate because the twoground circuits, GND for the integrated circuit and its substrate, andGNDO for the output driver transistors, are electrically separated.

In a similar manner, the driver control transistor 102 provides "ground"or more strictly, power supply voltage, control for the PMOS drivertransistor 100. The transistor 102 senses the local power supplyvoltage, V_(cc0), and provides feedback to reduce the conduction of thedriver transistor 100 when V_(cc0) has dropped low compared to theintegrated circuit's V_(cc) power supply. This provides V_(cc) bouncecontrol, over a problem which is not faced as often as the ground bounceproblem. Again it should be noted that transistor 102 operates becausethe two power supply circuits, for V_(cc) and V_(cc0), are electricallyseparated.

Finally, the unit 10A has the PMOS transistor 114 which is connected tothe slew rate control signal line 76. When its gate electrode is drivenlow, the PMOS transistor 114 improves the coupling of the gateelectrodes of the power transistors 100 and 101 to the source electrodesof the data transistors 109 and 108 respectively. This coupling speedsthe voltage transitions at the terminal 11.

FIG. 3 is the detailed circuit diagram of the multiplexer 43 of FIG. 1.The multiplexer 43 can be operated so that it can select data atdifferent logic levels with no significant loss in speed. Themultiplexer 43 has two data input terminals, the first connected to theoutput terminal of the latch 42 and the second connected to theinput/output terminal 11 through the line 44. The multiplexer 43 alsohas four control terminals, each respectively connected to the controlline 84, the control lines 62 and 63, and the control line 77, and anoutput terminal connected to the data input line 45 to the interior ofthe integrated circuit.

Four PMOS and NMOS transistors 120-123 in FIG. 3 function as a inverterwith a high impedance state for input data signals which operate at TTL(transistor-transistor logic) levels on the line 44 from the terminal11. The PMOS 122 transistor has its source connected to the V_(cc) powersupply and its drain electrode connected to the source of the PMOStransistor 120, which has its drain connected to the drain of the NMOStransistor 121. The NMOS transistors 121 has its source connected to thedrain of the NMOS transistor 123 which, in turn, has its sourceconnected to the ground power supply. The PMOS transistor 120 and NMOStransistor 121 have their gates commonly connected to the line 44. Thetransistors 120-123 are sized so that when the transistors 122 and 123are turned on, as explained below, the inverter switches states at +1.4volts as required by TTL circuits.

Likewise, four PMOS and NMOS transistors 124-127 function as a inverterwith a high impedance state for input data signals which operate at CMOS(complementary metal-oxide-semiconductor) logic levels on the line 44from the terminal 11. The PMOS 126 transistor has its source connectedto the V_(cc) power supply and its drain electrode connected to thesource of the PMOS transistor 124, which has its drain connected to thedrain of the NMOS transistor 125. The NMOS transistor 125 has its sourceconnected to the drain of the NMOS transistor 127 which, in turn, hasits source connected to the ground power supply. The PMOS transistor 124and NMOS transistor 125 have their gates commonly connected to the line44 (and gates of the transistors 120 and 121). The transistors 124-127are sized so that when the high impedance state transistors 126 and 127are turned on, as explained below, the inverter switches states at +2.5volts as required by CMOS logic circuits.

Similarly a third inverter with a high impedance state is formed by fourPMOS and NMOS transistors 128-131 in FIG. 3. The PMOS 130 transistor hasits source connected to the V_(cc) power supply and its drain electrodeconnected to the source of the PMOS transistor 128, which has its drainconnected to the drain of the NMOS transistor 129. The NMOS transistors129 has its source connected to the drain of the NMOS transistor 131which, in turn, has its source connected to the ground power supply. ThePMOS transistor 128 and NMOS transistor 129 have their gates commonlyconnected to the output terminal of the latch 42. The transistors128-131 are sized to operate at full CMOS logic levels between 0 and +5volts. When the high impedance state transistors 128 and 129 are turnedon, as explained below, the inverter switches states at +2.5 volts.

A fourth set of four transistors 132-135 operate as a level-shiftingoutput inverter having a high impedance state receiving the outputsignals of the previously described three inverters. The PMOS 134transistor has its source connected to a power supply at +3.5 volts andits drain electrode connected to the source of the PMOS transistor 132,which has its drain connected to the drain of the NMOS transistor 133.The NMOS transistors 133 has its source connected to the drain of theNMOS transistor 135 which, in turn, has its source connected to theground power supply. The PMOS transistor 132 and NMOS transistor 133have their gates commonly connected to the output terminals of thepreviously described three inverters, i.e., the commonly connecteddrains of the transistors 120 and 121, 124 and 125, and 128 and 129.

It should be noted that the PMOS transistors 132 and 134 are placed in aN-well separated from the N-well in which the other PMOS transistors areplaced. The N-well of the transistors 132 and 134 to be held at +3.5volts compared to the +5 volts for the N-well of the other PMOStransistors. This permits the output inverter when enabled to generatesignals at its output terminal, the commonly connected drains of thePMOS transistor 132 and NMOS transistor 133, between 0 and +3.5 volts.Thus the signals on the multiplexer's output terminal, which isconnected to the line 45, are compatible with the voltage levels for theinterior of the integrated circuit.

Inverters and NAND logic gates 136-141 decode the control signals on thecontrol lines 62 and 63 to enable one of the first three inverters. Thecontrol line 62 is connected to the input terminal of the inverter 136which has its output terminal connected to the gate of the PMOStransistor 128 and, through the inverter 141, the gate of the NMOStransistor 129. A logic "1" on the line 62 turns on the two transistors128 and 129 so that the input transistors 130 and 131 which have theirgates commonly connected to the data input terminal 145 becomeoperative. The third inverter is enabled to pass signals from the linefrom the latch 42 to the output inverter and to the output terminal andthe line 45.

If the signal on the control line 62 is a logic "0" then the thirdinverter is disabled and one of first two inverters are enabledresponsive to the control signal on the control line 63. A NAND gate 137has one of its two input terminals connected to the line 63; the otherinput terminal is connected to the output terminal of the inverter 136.The output terminal of the NAND gate 137 is directly connected to thegate of the PMOS transistor 122 and indirectly through the inverter 138to the gate of the NMOS transistor 123.

A second NAND gate 139 has one of its two input terminals connected tothe output terminal of the first NAND gate 137. The second inputterminal is connected to the output terminal of the inverter 136. Theoutput terminal of the NAND gate 139 is connected directly to the gateof the PMOS transistor 126 and, through an inverter 140, to the gate ofthe NMOS transistor 129.

Thus with a logic "0" on the line 62, a logic "1" on the control line 63turns on the transistors 122 and 123 to enable the first inverter. Theinput transistors 120 and 121 which have their gates commonly connectedto the data input line 44 become operative and the transistors 126 and127 of the second inverter are turned off. A logic "0" on the controlline 63 turns on the transistors 126 and 127 to enable the secondinverter and the transistors 122 and 123 of the first inverter areturned off. The second inverter's input transistors 124 and 125 whichhave their gates commonly connected to the data input line 44 now becomeoperative.

Before any of the output signals from the first, second or thirdinverters are passed to the output terminal 150, the output invertermust be enabled. A logic "0" on the control line 84 which is connectedto the gate of the PMOS transistor 134 and to the gate of the NMOStransistor 135 through an inverter 142 enables the output inverter. Ifthe control line 84 is at a logic "1," the transistors 134 and 135 areturned off and the output inverter is in a high impedance state.

The fourth control line 77 is connected to the gate of a NMOS transistor143 which has its drain connected to the power supply at V_(cc) and itssource connected to data input line 44. A logic "1" on the control line77 turns on the transistor 143 to provide a pullup for input/outputterminal through the line 44. In other words, if left alone theinput/output terminal 11 is pulled high toward V_(cc). On the otherhand, when enabled the transistor 143 operates weakly enough so that anysignal on the terminal 11 easily overpowers the action of the transistor143.

For input signals which are slow or sloppy, Schmitt triggers may be usedin place of the inverters formed by the transistors 120-123 and 124-127receiving the data signals from the terminal 11. The output signals of aSchmitt trigger have a sharp transition. FIG. 4 is a circuit schematicdiagram of a typical Schmitt trigger implemented in CMOS technology. Theinput signal and output signal lines are labeled with same referencenumbers as used in FIG. 3 along with the connections to the logicselecting the particular Schmitt trigger circuit. As stated above, theswitching point of each Schmitt trigger circuit can be adjusted byappropriately designing the operating parameters of the transistors inthe Schmitt trigger circuit. Thus one Schmitt trigger circuit canreceive TTL logic signals and the other CMOS logic signals.

FIG. 5 is a detailed circuit diagram of the 4- to-1 multiplexers 20, 30and 40 of FIG. 1. These multiplexers 20, 30 and 40 perform the dualfunctions of multiplexing and level-shifting with no substantial loss inspeed. Each of the multiplexers has four input data terminals 164-167,an output data terminal 172, and two control terminals 170 and 171.Control signals on the two terminals 170 and 171 selects which of theinput data terminals 164-167 is connected to the output terminal 172.

The four input data terminals 164-167 are connected respectively to asource/drain electrode of four NMOS transistors 154-157. The othersource/drain electrode of each of the transistors 154-157 are connectedin common to the input terminal of a inverter 152. The output terminalof the inverter 152 is connected to the input terminal of a secondinverter 151, which has its output terminal connected to the outputterminal 172 of the multiplexer. Also connected to the input terminal ofthe inverter 152 is the drain electrode of a PMOS transistor 153, whichhas its source electrode connected to the power supply terminal atV_(cc). The gate electrode of the transistor 153 is connected to theoutput terminal of the inverter 152.

Each of the two control signal terminals 170 and 171 are respectivelyconnected to two inverters in series. The terminal 170 is connected tothe input terminal of an inverter 162 which has an output terminalconnected to the input terminal of a second inverter 168. The terminal171 is connected to the input terminal of an inverter 163 which has anoutput terminal connected to the input terminal of a second inverter169.

The gate electrode of each of the four NMOS transistors 154-157 arerespectively connected to the output terminals of four NOR logic gates158-161. Each of the logic gates 158-161 have two input terminals whichare connected to the output terminals of the inverters 162, 163, 168 and169. The input terminals of the logic gates 158-161 are connected sothat only one of the logic gates generates a logic "1" to turn on one ofthe NMOS transistors 154-157 at a time. With one of the NMOS transistors154-157 turned on, one of the corresponding data signal input terminals164-167 is connected to the output terminal 172 through the inverters152 and 151.

The combination of connections to the output terminals of the inverters162, 163, 168 and 169 provides the basis for the multiplexing operationby the control signals at the terminals 170 and 171. For example, withtwo logic "1" 's on the terminals 170 and 171, the NOR gate 158generates a "1" output signal to turn on its NMOS transistor 154; theother logic gates 159-161 generate output "0" signals. Because of itsconnections to output terminals of the inverters 162 and 161, only theNOR gate 158 receives "0" signals at its input terminals. The other NORgates 159-161 are connected to other combinations of inverter outputterminals. With different control signals at the terminals 170 and 171,the other NOR gates 159-161 are selected to turn on their respectiveNMOS transistors 155-157.

The multiplexer shown in FIG. 5 also shifts the voltage levels of thedata input signals. As noted previously, the interior of the integratedcircuit operates between 0 to +3.5 volts. Thus most of the data signalsat the terminals 164-167 operate in a range from 0 to +3.5 volts (alongwith the output signals from the NOR logic gates 158-161). When one ofthe terminals 164-167 is selected by turning on the corresponding one ofthe NMOS transistors 154-157, which have a threshold voltage ofapproximately 1 volt, the data signal from the terminal ranges from 0 to+2.5 volts at the input terminal to the inverter 152. Of course, theinput data signal can range from 0 to +5 volts without changing theoperation of the multiplexers 20, 30 and 40.

The inverter 152, which operates between power supplies at 0 volts(ground) and +5 volts (V_(cc)), has an approximate switching point of+1.25 volts, midpoint in the range of voltages passed by the NMOStransistors 154-157 to the inverter's input terminal. The switchingpoint is determined by the design of the transistors which form theinverter 152, as is commonly done by integrated circuit designers. Theinverter's output signal switches between 0 and +5 volts. This signal inturn is switched by the second inverter 151, which also operates between0 and +5 volts fully between the two power supply voltages.

The PMOS transistor 153 has its drain electrode and gate electroderespectively connected to the input terminal and output terminal of theinverter 152. When the data signal at the input terminal of the inverter152 switches from zero to high, the transistor 153 helps pull up thevoltage at the inverter's input terminal to increase the switchingspeed. The transistor 153 acts as a weak pull-up so that when the datasignal switches low, the action of the transistor 153 is easilyoverpowered by the data signal to switch the inverter 152.

This design allows the multiplexing and level-shifting functions to beintegrated. The operational speeds of the multiplexers 20, 30 and 40 areoptimized.

FIG. 6 illustrates the details of the programming unit 50. The outputterminals of the unit 50 are labeled with the reference numbers of thecontrol lines 69-79 to which each of the output terminals are connectedas shown in FIG. 1. Each of the output terminals 69-79 are respectivelyconnected to one of several programming lines 230-239 and 249. Each ofthe lines 230-239 and 249 are connected to the source/drains of NMOStransistors 180-189 and 179 respectively. The other source/drains of thetransistors 179-189 are connected to ground. The gates of thetransistors are connected to 11 address lines from the decoder 51. Eachof the lines 230-239 and 249 are also connected to the source/drains ofNMOS transistors 190-199 and 209 respectively, which have their othersource/drains connected to a precharge programming line 200. The gatesof each of the transistors 190-199 are commonly connected to a prechargecontrol line 201.

Intersecting each of the lines 230-239 and 249 are two control lines202, 203 and three clock lines 204-206. At the intersection of eachprogramming line 230-239 and 249 and the control line 202 are antifuses210-219 and 259. In the drawings antifuses are symbolized by a circleand a bar. Furthermore, antifuses 220-229 and 269 are located at theintersection of each programming line 230-239 and 249 and the controlline 203. The programming line 232 also has antifuses 241-243 at itsintersection with the clock lines 204-206 and the programming line 235has antifuses 244-246 at its intersection with the clock lines 204-206.

Programming of the unit 50 is performed by connecting the 230-239 and249 and the terminals 70-79 and 69 (and their control lines) to thecontrol line 202, which is normally in a logic high state, to thecontrol line 203, which is normally in a logic low state, and to theclock lines 204-206, which carry different timing signals. Connection ismade by programming or "blowing" the antifuse at the intersectionbetween the conducting line and the control or clock line.

Antifuse programming is performed by first placing all the programminglines 230-239 and 249 into a precharge state. Through a signal on thecontrol line 201, all the transistors 190-199 and 209 are turned on toconnect the lines 230-239 and 249 to the precharge control line 200. Theline 200, at +5 volts, charges all the lines 230-239 and 249 to thisvoltage. Then the transistors 190-199 and 209 are turned off.

To program a selected antifuse, say, the antifuse 227, a large voltage,10 volts in this case, is placed across the targeted antifuse. Throughthe operation of the decoder 51 the transistor 187 is turned on toground the line 237. At the same time, through other decoder circuitrythe control line 203 is raised to a special programming voltage of +10volts. (The decoder circuitry and circuits for generating theprogramming voltage are not shown in the drawings. One skilled in theintegrated circuit design would know to design decoder and programmingvoltage circuits.) With 10 volts across the antifuse 227, the antifuseis programmed and the line 237 is connected to the control line 203 inthe low logic state. With the other lines 230-236, 238-239 and 249precharged to an intermediate voltage of +5 volts, the antifuses at theintersections of these lines do not experience the large programmingvoltage of 10 volts and hence remain unprogrammed. In this manner thecontrol lines 230-239 and 249 are set to a high logic state, i.e.,connection to the control line 202, or to a low logic state, i.e.,connection to the control line 203.

The conducting lines 232 and 235 may also be connected to the clocksignal lines 204-206 by programming the antifuses 241-246. Theseantifuses are programmed in same manner as described above. In thiscase, however, the clock lines 204-206 are raised to the programmingvoltage.

In this manner the programming unit can set the operation of theinput/output buffer circuit. For example, the control line 69 can be setlow by programming the antifuse 269. By programming one of the antifuses244-246, the control line 75 carries a clock signal so that the outputdriver stage 10 is periodically enabled. The terminal 11 is periodicallypulled low; otherwise the terminal 11 is part of an open circuit. Notethat the output driver stage 10 can be completely enabled or disabled byprogramming the antifuses 215 or 225 to set the control line 75 high orlow.

The structure of the particular antifuse used is disclosed in U.S. Ser.No. 07/672,501, entitled "IMPROVED METHOD OF FABRICATING ANTIFUSES IN ANINTEGRATED CIRCUIT DEVICE AND RESULTING STRUCTURE," filed Mar. 20, 1991by Pankaj Dixit et al. and assigned to the present assignee. However, itshould be evident to persons skilled in integrated circuit design thatother antifuses with different operational parameters, or other types ofelectrically programmable elements, such as floating gate MOStransistors, may be used in the input/output buffer circuit of thepresent invention.

For the convenient testing of the integrated circuit of which theinput/output buffers are a part, the present invention has large testpads which are placed at a corner of the integrated circuit die. With ascan-based technique for testing, such as the IEEE 1149.1 , thisclustering of test pads in one corner of the integrated circuit diepermits easy probing of the integrated circuit. Registration andalignment of the test probes are easier, especially for integratedcircuits having a large number of pins.

As shown in FIG. 9, bonding pads 303 are placed along the sides of thedie away from the corners, as in standard practice. Die corners aretypically not used because of likelihood of cracking during scribingoperations. The bonding pads 303 are the terminals for the input/outputbuffer circuits represented by rectangles 302 and for power circuits(Vcc and ground). The pads 303 provide locations where various bonds,such as wire or tab, are made for leads for connections to the externalsystem.

The test pads 301, which are at least one-and-half times larger than thebonding pads 303, are located in a corner 304 of the die and spacedapart for ease of testing and probing. These test pads 301 can be placedin other locations which are typically proscribed for bonding pads, suchas the center of the die, or on or in close proximity to the scribelines. These test pads are used during wafer testing of the dice in thewafer before scribing is performed. On the other hand, the bonding pads303 are packed for maximum usage of the tab or other interconnectiontechniques. Each of the test pads 301 corresponds to one of the powerbonding pads and to one of the bonding pads 303 for the IEEE 1149.1serial scan test, i.e., the bonding pads for TDI, TDO, TMS, and TCK.Each test pad 301 is connected to its corresponding bonding pad 303 by ametal interconnection line 305.

When a probe contacts a test pad 101, the probe also makes electricalcontact with the corresponding bonding pad 303 and connectedinput/output circuit (or power circuit). Thus with six probes, for TDO,TDI, TMS, TCK, power and ground, the integrated circuit can be tested byserial scanning without any contact to the bonding pads 303 of theintegrated circuit. As stated above, the testing of each integratedcircuit occurs when the integrated circuit is still part of a wafer. Yetno probe scores a bonding pad, which may damage the bonding pad andrender the integrated circuit damaged by the process of testing. Ofcourse, these test pads 301 may be used for other serial scan testingtechniques.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications and equivalentsmay be used. It should be evident that the present invention is equallyapplicable by making appropriate modifications to the embodimentsdescribed above. For example, while the invention has been described interms of CMOS technology, the present invention can be applied to othertechnologies, including bipolar and BiCMOS processes. Therefore, theabove description should not be taken as limiting the scope of theinvention which is defined by the metes and bounds of the appendedclaims.

What is claimed is:
 1. A multiplexer circuit for selecting signals froma plurality of input terminals to an output terminal responsive tosignals on at least one control terminal, said circuit comprisingaplurality of inverters, each inverter having an input node, an outputnode and means for enabling said inverter, each input node connected toinput terminal, said inverters including first and second invertersswitching at different voltage levels on an input node; logic meansconnected to said at least one control terminal and to said enablingmeans of each inverter for selecting one of said inverters responsive tosignals on said at least one control terminal; an output inverter havingan input node connected to said inverter output nodes; whereby saidmultiplexer selects signals operating at different logic levels fromsaid input terminals to said output terminal.
 2. The multiplexer circuitas in claim 1 wherein said input nodes of said first and secondinverters are connected to a first input terminal and said multiplexerhas a plurality of control terminals whereby said multiplexer can selectsignals operating at different logic levels on said first input terminalresponsive to signals on said control terminals.
 3. The multiplexercircuit as in claim 1 wherein said output inverter generates at saidoutput terminal signals operating at a different logic level from logiclevels of signals at said input terminals.
 4. The multiplexer circuit asin claim 1 wherein each inverter comprises two NMOS and two PMOStransistors having source/drain electrodes connected in series, anoutput node between said NMOS transistors and said PMOS transistors, afirst NMOS transistor and a first PMOS transistor having gate electrodesforming an input node connected to an input terminal, and a second NMOStransistor and a second PMOS transistor having gate electrodes connectedto said logic means for enabling said inverter.
 5. The multiplexercircuit as in claim 1 wherein each inverter comprises a Schmitt triggercircuit.
 6. The multiplexer circuit as in claim 4 wherein the parametersof said two NMOS and two PMOS transistors of each inverter aredetermined to set switching voltage levels for said inverter.
 7. Themultiplexer circuit as in claim 6 wherein said inverters operate betweena first and second voltage supply to determine the output voltage swingof said inverters at each inverter output node and said output inverteroperates between a third and fourth voltage supply to determine theoutput voltage swing of said output inverter at said output terminal,said output voltage swing of said inverters being different from saidoutput voltage swing of said output inverter.
 8. A multiplexer circuitfor selecting signals from a plurality of input terminals operating at afirst set of logic levels to an output terminal operating at a secondset of logic levels responsive to signals on at least one controlterminal, said circuit comprisinga plurality of MOS transistors, eachtransistor having a first source/drain electrode connected to an inputterminal, a second source/drain electrode and a gate electrode; logicmeans connected to a gate electrode of each MOS transistor for turningon one of said MOS transistors responsive to signals on said at leastone control terminal, said logic means operating at said first set oflogic levels; a first inverter having an input node connected to saidsecond source/drain electrodes of said MOS transistors, and an outputnode, said first inverter having a switching point approximately at themidpoint of voltages passed by said MOS transistors to said input node,and said output node of first inverter operating at said second set oflogic levels; a second inverter having an input node connected to saidoutput node of said first inverter and an output node connected to saidoutput terminal, said second inverter having a switching pointapproximately one-half the voltage swing of said second set of logiclevels and said output node of second inverter operating at said secondset of logic levels.
 9. The multiplexer circuit as in claim 8 furthercomprising a PMOS transistor having a source electrode connected to afirst power supply, a drain electrode connected to said input node ofsaid first inverter and a gate electrode connected to said output nodeof said first inverter whereby said PMOS transistor helps pull up asignal from a selected input terminal toward a voltage of said firstpower supply.
 10. The multiplexer circuit as in claim 9 wherein said MOStransistors comprise NMOS transistors.